If you are writing in PAST tense, it wouldn't be *she is*—it would be *she was*. This means you can't use *she's* for your past tense sentence if you mean to say *she was*.

I had or I would = I'dI'd is not used for I did, I could, I should, etc.

So, ultimately, 's (apostrophe s) means either is or has while 'd (apostrophe d) means did or would.

Contractions are good, especially when making the voice of your character sound natural. However, you need to use them correctly. Some writers have issues with contractions when they write in different tenses—past tense vs present tense.

Additionally, the contraction *they're* is *they are* (present tense) and not *they were* (past tense) — contractions are good, but need to be used correctly within the tense you are writing.

GrammarGirl.com says: "So if you find yourself using a contraction with an apostrophe plus a 'd,' consider spelling it out instead. Although your sentence might be perfectly clear to you, it might not be so clear to someone reading it for the first time. Contractions are useful, especially when you’re writing informally. But beware of potentially confusing or ambiguous contractions and try to avoid those that sound awkward."